1. Field of the Invention
This invention relates to a process and an apparatus for treating waste for an anaerobic digestion of the biogenic organic constituents of the garbage, particularly the source separated organic waste, the wet waste, the residual waste and the commercial wastes.
2. Description of the Prior Art
The pretreatment and sorting of waste particularly of domestic and commercial waste have previously been effected almost exclusively by sorting means. The various fractions were intended to be separated without a substantial change of their moisture content. Conventional sorting comprises various combinations of disintegrating means, sieves, air-classifiers and means for hand-picking. An example of such means is disclosed in German Patent Specification 30 37 714.
It has also been proposed to process waste materials by a hydraulic separation, which may be performed as an independent process or as a process step in a sorting plant of the kind described hereinbefore.
For instance, published German Application 33 25 504 describes a process of treating and partly recycling domestic waste, bulky refuse, commercial waste material consisting mainly of garbage packaging material, and sewage sludge, as well as means for carrying out the process, in which the waste materials are initially disintegrated and separated into three sieve fractions. It has been proposed to fractionate the medium size fraction in that it is fed to a water-filled tub, in which heavy inorganic substances are intended to subside to the bottom whereas the light-weight substances rise to the surface so that they can be skimmed off and can be composted or subjected to anaerobic digestion.
A similar arrangement is described in published German Application 38 36 379, which relates to a process of treating the wet waste for the digestion of its organic content together with sewage sludge and an apparatus for a mechanical separation of organic and inorganic components of the refuse. In that case the medium size fraction which has been sieved off is treated by an apparatus in which spiked rollers are used to separate the organic and inorganic components.
Published German Application 35 00 132 describes the recovery and separation of organic components from the garbage in a process in which the light-weight components which have not been disintegrated are pneumatically separated from the mixed garbage by an air blast and are fed to a water pool, in which the heavy substances are caused to subside whereas the substances floating on the surface are carried by a weak current to discharge means.
But a separation of waste materials in stagnant or only slightly moving water will produce only unsatisfactory results. The settling velocity of the inorganic materials, particularly of the fines, is only low, and it is impossible to separate plastic materials with a density similar to organic constituents. For this reason it is taught in published German Application 35 00 132 that the waste materials which have risen to the surface should be boiled so that the structure of the organic substances will be destroyed to such a degree that they can be forced through a fine sieve and can be separated from the heat-resistant components, such as the plastics. But that process will require expensive equipment and has a high energy consumption.
A further disadvantage resides in that a very large amount of water in comparison with the amount of solids will be required for a separation in stagnant or slightly moved water and connection with the processes discussed no information has been furnished how the water which is laden with dissolved or suspended particulate organic and inorganic substances should be utilized or purified.
For preprocessing waste materials prior to a biotechnological process it is useful, particularly if a production of biogas is intended, to reduce the size of the organic substances before, after or during their separation in order to increase the reaction rate. In accordance with published German Application 38 36 379 a ball mill is used to disintegrate the organic fractions after their separation. It has been described in Published German Application 38 36 379 that the organic substances by disintegration in the ball mill will form a pulp so that the organic substances which have thus been disintegrated can then be utilized only in conventional biogas-plants with single phase digestion, as known from sewage sludge digestion or in agricultural biogas production.
The problems and operational restrictions involved in the operation of single-stage biogas-reactors have been explained in detail in the literature. It is known that much more efficient biogas-reactor system can be provided if the dissolution (hydrolysis) of the organic solids is effected prior to the methane production in a separate reactor system from which only the dissolved organic substances are fed into the methane reactor. Efficient reactor systems for the production of methane from dissolved organic substances are already commercially available.
But two-stage-digestion necessitates a highly efficient separation of organic from inorganic waste constituents. Those said organic substances need to be disintegrated in a way that their fibrous structure is preserved. This cannot be accomplished by the disintegrating means which have previously been used for size reduction of waste materials, particularly by the above-mentioned ball mill. But such a disintegration of the organic waste materials will inevitably result in a pulp so that it is difficult to remove the undissolved organic solids and that disintegration will unfavorably always involve an excessive reduction in size of the inorganic solids so that heavy metals will undesirably be entrained.
The above-mentioned sorting means are only of limited use for a separation of the non-digestable substances (glass, plastic sheeting and plastic containers, metals, stones, sand) because such substances are intimately blended with and adhesively bonded to the moist organic substances. For this reason said sorting and disintegrating means and the treating processes in which such means are employed cannot be used for a processing of waste materials to be subjected to biotechnological utilization.
A pulper may be used for a wet processing of waste materials without a need for previously sorting or disintegrating them. This has been described, e.g. in German Patent Specification 20 47 006. Mixed waste materials are continuously fed into the pulper, where they are simultaneously defibrated and disintegrated. Slurry is continuously withdrawn as a suspension through a perforate sieve. The defibration affects the biogenic organic waste materials, particularly the paper and cardboard materials, kitchen and yard waste. The impeller is so designed that all other substances will be broken or cut. Glass is to be crushed and aluminum is to be compacted to balls, which owing to relatively large perforation holes of the sieve are pumped off together with the fiber-containing slurry and are separated in subsequent process stages. Because the inorganic and metallic fractions are separated from the organic substances by hydrocyclones the pulper must be operated at a low solids concentration. Those non-digestable materials which cannot be defibrated or disintegrated pass through an opening in the bottom into a special separating and cleaning system, in which the fibers are separated for being recycled to the pulper.
In that case the disintegration of the inorganic non-digestable substances has a strong adverse effect on the quality of the fibrous organic substances because finely divided heavy metals will be released as a result of the disintegration and will enter the final product so that its use for agricultural soil amendment will be restricted or even precluded. A special disadvantage is involved in the crushing of glass because the sharp-edged particles of broken glass cause intense abrasion in pumps and pipes as well as congestions in tanks and pipes and may clog them.
A special problem arising in the processing of waste materials in a pulper is due to plastic materials in the form of large sheets and containers. In the processing of waste paper the nonbiological substances contained therein have such characteristic properties that cords and wires will form a rope, which is slowly withdrawn by means of a winch. But even in the processing of waste paper the content of the non-biological substances may be so high that that method will not be satisfactory. German Patent Specification 32 25 026 describes for that purpose a raking device which can allegedly be used for a reliable and continuous removal of coarse and heavy waste materials with a small expenditure of operating labor.
In the processing of waste, particularly of residual and commercial waste materials, it must be assumed that the sheet-like components (plastic sheeting) and the bulky ones (containers, bottles) cannot form a rope. For this reason published German Application 24 06 404 relating to a process of recovering recyclable components from waste material proposes to entrain the plastics out of the pulper by a partial stream of the suspension and to effect a separation into a fibre-containing suspension and non-biodegradable substances in a different, additional apparatus. In that case the suspension in the pulper is also required to operate at low solids concentration.
European Patent 0 286 100 discloses a process for the pretreatment and anaerobic digestion of biogenic organic waste and an apparatus for carrying out the process. In that process those fractions which are not biodegradable are first removed from the raw waste material. In a pretreating stage the biogenic organic fraction of the waste material is then softened by an addition of water or liquid waste material and is subjected at the same time to combined hydraulic and mechanical shearing actions to disintegrate the waste material whereas its fibrous structure is substantially preserved. This results in a raw suspension, which contains 3 to 15% by weight organic solids. In a succeeding alkaline pretreatment the raw suspension is adjusted to an alkaline state by adding chemicals and is heated to 40.degree. to 60.degree. C. and is held at that temperature for 2 to 12 hours. The suspension withdrawn from the alkaline pretreatment is subsequently divided in a solid-liquid separating stage into a liquid stream, which contains the water-soluble biological substances, and a solids-laden stream, which contains 20 to 50% by weight organic solids. The liquid stream is subsequently subjected to a methane fermentation and the solids-loaden stream is subjected to an anaerobic hydrolysis of solids under acid conditions and at a temperature in the mesophilic or thermophilic range and the resulting hydrolyzate is subjected to the methane fermentation. A pulper which can be used for the pretreatment has been disclosed in published German Application 37 11 813 of the same applicant.
The apparatus described there and its mode of operation for separating an organic fibrous fraction cannot be used to satisfactorily prepare said fibrous fraction for the succeeding anaerobic treatment and to recover the non-digestable substances in a way that they can be recycled respectively disposed of. That disadvantage will be the more significant the higher the content of the non-digestable substances in the waste material is to be processed because only a small part of the nondigestable substances floating in the suspension can be skimmed off during the pulping operation. The separation of the heavy substances is also unsatisfactory, particularly if the solids concentration in the suspension should be as high as possible. Besides, the non-digestable substances are accompanied by adhering fibrous substances, so that the materials must be aftertreated before they can be disposed of or recycled.
As a result, non-digestable substances cannot be removed unless the pulper is operated for a longer time than would be necessary only for the pulping and defibration of the organic substances. But the longer duration of the operation performed at high speed will increase the power consumption and the abrasion.
Whereas the separating effect could be improved in that the solids concentration in the suspension is decreased, this would undesirably require more expensive equipment and would increase energy requirement for the succeeding stages, particularly for the solid-liquid separation and the hydrolysis of solids.